1. Field of the Invention
The present invention pertains to the area of jet engines and more particularly to engines whose fan is integral with a drive shaft which is supported by a first bearing and a second bearing.
Said turbo-jet engine, from upstream to downstream in the direction of the exhaust stream, comprises a fan, one or more compressor stages, one or more turbine stages and an exhaust pipe. The fan comprises a rotor provided with blades on its circumference which, when caused to rotate, drive air into the turbo-jet engine. The fan rotor is supported by the low pressure rotor shaft of the engine. It is centred on the axis of the jet engine by a first bearing which is upstream from a second bearing joined to the fixed structure, to the intermediate casing in particular.
In the remainder of the description, insofar as the fan is mounted integral with the compressor shaft, which is the shaft of the low pressure rotor in a double-body engine, this shaft shall be designated by the single term compressor shaft.
The first bearing is supported by a support piece forming an enclosure around the compressor shaft, oriented downstream from the first bearing and being attached to a fixed structure of the jet engine. The second bearing is supported by a support piece that is also attached to a fixed structure of the jet engine.
It may happen that a fan blade is accidentally lost. The result is substantial imbalance on the compressor shaft which leads to loads and vibrations on the bearings, transmitted via their support pieces to the fixed structures of the jet engine which may therefore be damaged.
2. Description of the Related Art
To prevent the risk of too extensive damage to the jet engine, the structure may be over-dimensioned or, as proposed by patent FR 2 752 024, an uncoupling system for the first bearing may be provided. The support piece of the first bearing is attached to the structure of the jet engine by so-called shearing screws which contain a weakened portion causing their rupture if the loads applied are too high. Therefore, when an imbalance occurs on the compressor shaft, the stresses induced on the first bearing are transmitted to the shearing screws which break, uncoupling the support piece of the first bearing from the structure of the jet engine. According to one embodiment, the support of the second bearing is associated with the support of the first bearing, accompanying it in the event of uncoupling. The stresses caused by the imbalance are then no longer transmitted to the fixed structure of the jet engine by these support pieces.
However, after uncoupling of the bearings, the fan continues to rotate and it is possible that the compressor shaft is no longer able to rotate about its axis undergoing major displacement which may damage the fixed structure of the jet engine. Patent FR 2 752 024 provides in this case, on the fixed structure of the jet engine, for a ridge surrounding the support piece of the two bearings and acting as axial stop and as emergency bearing.
However, in this case, the emergency bearing is radially far distant from the original bearings, which completely modifies the distribution of stresses on the various engine parts.
Finally, since the second bearing is able, in some cases, to ensure the maintaining of the compressor shaft after uncoupling of the first bearing, it is not necessary for both bearings to be systematically uncoupled simultaneously, as it is always preferable to maintain the jet engine in configurations that are the closest to normal.